A British mother detained in Iran over allegations she plotted to overthrow the government is being subjected to psychological torture from “the whole armoury of the state”, her supporters believe.

Nazanin Zaghari-Ratcliffe, who is entering her 683rd day in prison, has been the victim of “psychological abuse” which meets United Nations criteria for torture, according to campaigners calling for her release.

The dual UK-Iranian national, who was detained on April 3 2016 while visiting her parents with her daughter Gabriella, is said to have spent eight-and-a half months in solitary confinement before moving to a general ward.

The 39-year-old’s treatment is also said to have included repeated threats to take away her daughter or against others supporting her during the initial months after she was detained, as well as denial of visits, phone calls and medical services, and broken promises of release.

The charity Redress, which has written to the UN special rapporteur on torture on behalf of the Free Nazanin campaign, said evidence “strongly suggests” the treatment of the mother-of-one amounts to torture.

She denies the allegations, and her family believe she is being used by the Iranian authorities as a diplomatic “tool of pressure”.

Jailed British mother Nazanin Zaghari-Ratcliffe with her husband Richard Ratcliffe and their daughter Gabriella (Family Handout/PA)

Redress said her treatment was being inflicted to coerce the British Government into securing a deal for her release and to force her to confess or provide information about others.

It has written to Professor Nils Melzer referring to a “state-orchestrated campaign” involving agencies including Iran’s revolutionary guard, judiciary and state television service.

The letter reads: “While it has serious effects on her physical health, the combined effect of this campaign has been to exert consistently maximum psychological pressure on Mrs Zaghari–Ratcliffe, to demoralise her and put her in a situation of complete powerlessness.

“The treatment has been inflicted on her over an extended period of time, and while she was, and continues to be, at her most vulnerable, a recent mother, in prison abroad and away from her family in the United Kingdom, without consular access and without allowing her family to visit.”

Photos of Richard Ratcliffe and his wife Nazanin Zaghari-Ratcliffe, who has been jailed in Iran, on display at their home in north London (John Stillwell/PA)

Her husband, Richard Ratcliffe, said he spoke to his wife on the phone on Tuesday, and that she was “not as bleak as she was a few weeks back, but she’s very up and down”.

The 43-year-old, from Hampstead, north London, said: “It’s very hard for both of us to be hopeful at this point. We were obviously very hopeful at Christmas (for her release) and it didn’t happen.

“It’s always good to hold on to the ‘maybe’, but also part of us putting in the submission now is because it feels like we’re just being gamed.”

He said there had been more than half a dozen direct assurances and less clear indications his wife would be released, and that he had resumed campaigning because “we can’t keep waiting in the doorway of our dreams, waiting for the governments to deliver”.

The former charity worker is said to be experiencing post-traumatic stress disorder and depression, giving rise to “darkly negative feelings”,”uncontrollable bouts of anger”, and suicidal tendencies.

She has also experienced difficulty walking and suffered extreme hair and weight loss.